Tubular members such as drill pipe, tubing pipe and casing used in the oil and gas industry (herein referred to as "pipe sections") are joined at their ends by threaded connections. Power tongs and back-up tongs are used to make up (i.e. join) and break out (i.e. disconnect) these threaded connections. The back-up tong grips the lower stationary pipe section while the power tong grips the adjoining upper pipe section and rotates it to provide a fluid-tight seal during make up of the drill string, and to break out the threaded connection during disassembly of the drill string.
Several dies or die inserts engage the outer cylindrical or peripheral surface of each pipe section during the gripping action. FIG. 1 shows one conventional design of a jaw frame 10 which carries two flat dies 11 with teeth for biting the pipe section's peripheral surface. The jaw frame 10 has a cam follower or jaw roller 12 which travels along a cam surface of a power tong's rotary gear in a known manner for camming the dies 11 against the pipe section. Typically two or more jaw frames are required to properly grip a pipe section. The flat die shown is popular because it is relatively inexpensive and may be replaced when worn out.
It is desireable that the jaws' pipe engaging portions (for example, the die inserts) spaced about a pipe section provide an even or uniform bite on the pipe section during use. The center of each pipe engaging portion should be generally tangential to the pipe section during initial contact and even upon applying torque to the pipe section so that a maximum number of jaw teeth squarely engage the pipe section's peripheral outer surface. In FIG. 1 the die inserts 11 are fixed in the jaw 10 to properly bite a 5.5 inch outer diameter ("OD") pipe, indicated by reference numeral 14. A disadvantage of the prior art jaw design is that such desired contact is lost for different sized pipe sections, such as the 4.75 inch OD pipe section indicated by 15. Hence, another tong is needed or the entire jaw assembly should be replaced to provide the die inserts with the desired orientation to the new pipe section 15. Such replacement is expensive and time consuming, and therefore not practical for typical rig operators. If the existing jaw is used without being replaced, then the die teeth may unduly damage the pipe section's peripheral surface, requiring early replacement or repair of the pipe section to avoid buckling or failure during use.
A related problem with prior art jaw frame designs is that they do not adequately address uneven wear of the tool joints of the drill string. Even if a jaw assembly provides an even bite upon initial contact with one pipe section, it will not necessarily bite properly on another pipe section of a drill string because the OD on each pipe section is not necessarily uniform due to wear, thus reducing die-to-pipe contact. Upon the application of torque the die teeth which remain in contact with the pipe section tend to dig deeper into the pipe wall to transmit a required amount of torque for make up or break out, thereby further accelerating wear on the pipe sections.
Yet another problem with conventional jaw assemblies, particularly in back-up tongs, is that the hydraulic cylinder which pushes the jaw assembly into contact with a pipe section tends to relax somewhat during use when a small volume of hydraulic fluid leaks out of the cylinder, resulting in a small but unwanted release of clamping force by the dies on the pipe section. The pipe section may therefore slip during make up or break out operations and damage the tool joint's peripheral wall, or the operator must manually apply more hydraulic pressure than is otherwise necessary to compensate for such relaxation.
Prior references such as U.S. Pat. No. 5,172,613 (Wesch, Jr.), U.S. Pat. No. 5,167,173 (Pietras) and U.S. Pat. No. 4,437,363 (Haynes) attempt to address some of the above problems, but the proposed solutions are flawed, and none of the references are able to overcome all of the above problems.
What is therefore desired is a novel jaw assembly which overcomes the limitations and disadvantages of existing assemblies. Preferably, the jaw assembly should have individual die holders each of which is independently capable of squarely aligning a pipe engaging face or die insert with any one of a variety of pipe section sizes, and in particular the pipe gripping face should align itself substantially tangentially with the peripheral surface of a given pipe section upon engaging it. The die holder of the jaw assembly of the present invention should also provide an independent or secondary camming action to enhance its grip on the pipe section during make up and break out operations, and provide an even or substantially uniform bite about the pipe section. The die holder should be capable of holding and using conventional and relatively inexpensive flat die inserts.